Recent years have seen the development of biometric authentication technology for determining whether or not to authenticate individuals by using biometric information, such as fingerprints or palmprints. The biometric authentication technology has been widely used ranging from large-scale systems for a large number of registered users, such as a border control system and a system using citizen identification numbers, to specific-personal-use apparatuses, such as computers and mobile terminals.
For example, when the fingerprint of any finger is used as biometric information, a biometric authentication device acquires, as an input biometric image, a biometric image representing the fingerprint. The biometric authentication device compares the input biometric information, which is the user's fingerprint represented in the input biometric image, with registered biometric information, which is a fingerprint represented in a biometric image of a registered user. Upon determining that the input biometric information and the registered biometric information match each other on the basis of a result of the comparison processing, the biometric authentication device authenticates the user as a registered user having a legitimate right. The biometric authentication device permits the authenticated user to use an apparatus in which the biometric authentication device is incorporated or another apparatus connected with the biometric authentication device.
When a sensor reads biometric information (such as a fingerprint or palmprint) found on the surface of a specific portion of the human body, there are cases in which part in the biometric information is distorted by the condition or characteristics of the surface of the portion of the user or motion of the portion during the reading. For example, when the user excessively presses his or her finger against the sensor, fingerprint ridges are flattened to cause distortion in the fingerprint represented in the biometric image. Thus, when a biometric image representing partly distorted biometric information is used for biometric authentication, the biometric authentication device may fail to accurately extract features of the biometric information. This may result in a decline in the authentication accuracy.
Accordingly, there is a proposed technology for detecting and/or correcting a distortion in biometric information in a biometric image.
For example, Japanese Laid-open Patent Publication No 2007-272568 discloses a technology in which a group of feature points selected from one of two pieces of feature information and feature points included in the other one of the two pieces of feature information and corresponding to the selected feature points is set as position-alignment candidates and the amount of position correction with which pieces of position information of the feature points in the group generally match each other is determined.
Japanese Laid-open Patent Publication No. 04-324582 discloses a technology in which the amount of position correction is determined based on the number of comparisons and the amount of position displacement between a fingerprint obtained during registration and a fingerprint obtained during each of the comparisons.
In addition, Japanese Laid-open Patent Publication No. 09-282458 discloses an image verification device that aligns an input image of a fingerprint with a reference image after correcting a geometric distortion in the input image. This image verification device determines a reference position in a block of interest in an input image on the basis of the distance between the image center of the input image and the block of interest and determines correlations with the input image while displacing, in the reference image, influence blocks corresponding to the block of interest. The image verification device then corrects the reference position by using the amount of displacement of the influence block having the largest correlation.
Additionally, Japanese Laid-open Patent Publication No. 2008-310814 discloses a technology in which a deformable mesh is constructed with respect to a first fingerprint feature point and the state of the deformable mesh is transformed to obtain a distortion-compensated first fingerprint, and the distortion-compensated first fingerprint is compared with a second fingerprint.